Chytridiomycota

Chytridiomycota
A spizellomycete
Scientific classification
Kingdom: Fungi
Division: Chytridiomycota
Hibbett et al. (2007)
Classes/Orders

Chytridiomycota is a division in the kingdom Fungi. The name is derived from the Greek chytridion, meaning "little pot", describing the structure containing unreleased zoospores. In older classifications, chytrids (except the recently established order Spizellomycetales) were placed in the class Phycomycetes under the subphylum Myxomycophyta of the kingdom Fungi. Previously, they were placed in the Mastigomycotina as the class Chytridiomycetes.[1] Also, in an older and more restricted sense (not used here), the term "chytrids" referred just to those fungi in the order Chytridiales. Here, the term “chytrid” will refer only to members of Chytridiomycota.[2] The chytrids have also been included among the Protoctista,[3] but are now regularly classed as fungi. Chytrids are one of the early diverging fungal lineages and are saprobic, degrading refractory materials such as chitin and keratin, or acting as parasites.[4] Their membership in kingdom Fungi is demonstrated with chitin cell walls, a posterior whiplash flagellum, absorptive nutrition, use of glycogen as an energy storage compound, and synthesis of lysine by the α-amino adipic acid (AAA) pathway.[2][5] There are over 750 chytrid species distributed among 7 orders.[6] There has been a significant increase in the research of chytrids since the discovery of Batrachochytrium dendrobatidis, the causal agent of chytridiomycosis.[7][8]

Life cycle & body plan

Chytridiomycota are unusual among the Fungi in that they reproduce with zoospores.[4][9] For most members of Chytridiomycetes, sexual reproduction is not known. Asexual reproduction occurs through the release of zoospores (presumably) derived through mitosis.[4] Where it has been described, sexual reproduction of Chytridomycetes occurs via a variety of methods. It is generally accepted that the resulting zygote forms a resting spore, which functions as a means of surviving adverse conditions.[4] In some members, sexual reproduction is achieved through the fusion of isogametes (gametes of the same size and shape). This group includes the notable plant pathogens Synchytrium. Some algal parasites practice oogamy: a motile male gamete attaches itself to a nonmotile structure containing the female gamete. In another group, two thalli produce tubes that fuse and allow the gametes to meet and fuse.[4] In the last group, rhizoids of compatible strains meet and fuse. Both nuclei migrate out of the zoosporangium and into the conjoined rhizoids where they fuse. The resulting zygote germinates into a resting spore.[2] Sexual reproduction is common and well known among members of the Monblepharidomycetes. Typically, these chytrids practice a version of oogamy: the male is motile and the female is stationary. This is the first occurrence of oogamy in kingdom Fungi.[5] Briefly, the monoblephs form oogonia, which give rise to eggs, and antheridia, which give rise to male gametes. Once fertilized, the zygote either becomes an encysted or motile oospore,[4] which ultimately becomes a resting spore that will later germinate and give rise to new zoosporangia.[5] Upon release from the germinated resting spore, zoospores seek out a suitable substrate for growth using chemotaxis or phototaxis. Some species encyst and germinate directly upon the substrate; others encyst and germinate a short distance away. Once germinated, enzymes released from the zoospore begin to break down the substrate and utilize it produce a new thallus. Thalli are coenocytic and usually form no true mycelium (having rhizoids instead). Chytrids have several different growth patterns. Some are holocarpic, which means they only produce a zoosporangium and zoospores. Others are eucarpic, meaning they produce other structures, such as rhizoids, in addition to the zoosporangium and zoospores. Some chytrids are monocentric, meaning a single zoospore gives rise to a single zoosporangium. Others are polycentric, meaning one zoospore gives rise to many zoosporangium connected by a rhizomycelium. Rhizoids do not have nuclei while a rhizomycelium can.[5] Growth continues until a new batch of zoospores are ready for release. Chytrids have a diverse set of release mechanisms that can be grouped into the broad categories of operculate or inoperculate. Operculate discharge involves the complete or incomplete detachment of a lid-like structure, called an operculum, allowing the zoospores out of the sporangium. Inoperculate chytrids release their zoospores through pores, slits, or papillae.[4]

Brief taxonomic history

Species of Chytridiomycota have traditionally been delineated and classified based on development, morphology, substrate, and method of zoospore discharge.[3][4] However, single spore isolates (or isogenic lines) display a great amount of variation in many of these features; thus, these features cannot be used to reliably classify or identify a species.[3][4][10] Currently, taxonomy in Chytridiomycota is based on molecular data, zoospore ultrastructure and some aspects of thallus morphology and development.[3][10]

Habitats

Chytrids are aquatic fungi, though those that thrive in the capillary network around soil particles are typically considered terrestrial.[3][4] The zoospore is primarily a means of thoroughly exploring a small volume of water for a suitable substrate rather than a means of long range dispersal.[11] Chytrids have been isolated from a variety of aquatic habitats, including peats, bogs, rivers, ponds, springs, and ditches, and terrestrial habitats, such as acidic soils, alkaline soils, temperate forest soils, rainforest soils, arctic and Antarctic soils.[3][4] This has led to the belief that many chytrid species are ubiquitous and cosmopolitan.[3][4] However, recent taxonomic work has demonstrated that this ubiquitous and cosmopolitan morphospecies hide cryptic diversity at the genetic and ultrastructural levels.[12][13] One of the least expected terrestrial environments the chytrid thrive in are periglacial soils.[14] The population of the Chytridiomycota species are able to be supported even though there is a lack of plant life in these frozen regions due to the large amounts of water in periglacial soil and pollen blowing up from below the timberline. It was first thought aquatic chytrids (and other zoosporic fungi) were primarily active in fall, winter, and spring.[4] However, recent molecular inventories of lakes during the summer indicate that chytrids are an active, diverse part of the eukaryotic microbial community.[15]

Ecological functions

Batrachochytrium dendrobatidis

Scanning electron micrograph of a frozen intact zoospore and sporangia of the chytrid fungus Batrachochytrium dendrobatidis, CSIRO
Main article: Chytridiomycosis

The chytrid Batrachochytrium dendrobatidis is responsible for chytridiomycosis, a disease of amphibians. Discovered in 1998 in Australia and Panama this disease is known to kill amphibians in large numbers, and has been suggested as a principal cause for the worldwide amphibian decline. Outbreaks of the fungus were found responsible for killing much of the Kihansi Spray Toad population in its native habitat of Tanzania,[16] as well as the extinction of the golden toad in 1989. The process leading to frog mortality is thought to be the loss of essential ions through pores made in the epidermal cells by the chytrid during its replication.[17]

Cures to this devastating disease exist, but they are currently limited to captive individuals. [18] Further experimentation with symbiotic growth-inhibiting bacteria may shed light onto the salvation of wild populations.

Other parasites

Chytrids mainly infect algae and other eukaryotic and prokaryotic microbes. The infection can be so severe as to control primary production within the lake.[5][19] It has been suggested that parasitic chytrids have a large effect on lake and pond food webs.[20] Chytrids may also infect plant species; in particular, Synchytrium endobioticum is an important potato pathogen.[21]

Saprobes

Arguably, the most important ecological function chytrids perform is decomposition.[3] These ubiquitous and cosmopolitan organisms are responsible for decomposition of refractory materials, such as pollen, cellulose, chitin, and keratin.[3][4] There are also chytrids that live and grow on pollen by attaching threadlike structures, called rhizoids, onto the pollen grains.[22] This mostly occurs during asexual reproduction because the zoospores that become attached to the pollen continuously reproduce and form new chytrids that will attach to other pollen grains for nutrients. This colonization of pollen happens during the spring time when bodies of water accumulate pollen falling from trees and plants.[4]

Fossil record

The earliest fossils of chytrids are from the Scottish Rhynie chert, a Devonian-age lagerstätte with anatomical preservation of plants and fungi. Among the microfossils are chytrids preserved as parasites on rhyniophytes. These fossils closely resemble the modern genus Allomyces.[23] Holocarpic chytrid remains were found in cherts from Combres in central France that date back to the late Visean. These remains were found along with eucarpic remains and are ambiguous in nature although they are thought to be of chytrids.[24] Other chytrid-like fossils were found in cherts from the upper Pennsylvanian in the Saint-Etienne Basin in France, dating between 300 and 350ma.[25]

In fictional media

The novel Tom Clancy's Splinter Cell: Fallout (2007) features a species of chytrid that feeds on petroleum and oil-based products. In the story the species is modified using nuclear radiation, to increase the rate at which it feeds on oil. It is then used by Islamic extremists in an attempt to destroy the world's oil supplies, thereby taking away the technological advantage of the United States.[26]

References

  1. ↑ The Fungi: An Advanced Treatise Vol IVB A Taxonomic Review with Keys: Basidiomycetes and Lower Fungi. 1973. Edited by Ainsworth, Sparrow & Sussman. Academic Press: New York.
  2. 1 2 3 Alexopoulos CJ, Mims CW, Blackwell M. 1996. Introductory Mycology. 4th edition. John Wiley & Sons, Inc.
  3. 1 2 3 4 5 6 7 8 9 Barr DJS. 1990. Phylum Chytridiomycota. In: Handbook of Protoctista. Eds Margulis, Corliss, Melkonian, & Chapman. Jones & Barlett, Boston. Pgs. 454-466.
  4. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Sparrow FK. 1960. Aquatic Phycomyetes. The University of Michigan Press:Ann Arbor. 2nd edition
  5. 1 2 3 4 5 Kendrick, Bryce. 2000. The Fifth Kingdom. 3rd edition Focus Publishing: Newburyport, MA.
  6. ↑ http://bama.ua.edu/~nsfpeet/
  7. ↑ Blackwell M. 2011. The Fungi: 1,2,3 … million species? American Journal of Botany 98:426-438.
  8. ↑ Longcore JE, AP Pessier & DK Nichols. 1999. Batrachochytirum dendrobatidis gen. et sp. nov., a chytrid pathogenic to amphibians. Mycologia 91(2):219-227.
  9. ↑ Hibbett et al. 2007. A higher-level phylogenetic classification of the Fungi. Mycologia 111(5): 509-547.
  10. 1 2 Blackwell WH, PM Letcher, & MJ Powell. 2006. Thallus development and the systematics of Chytridiomycota: an additional developmental pattern represented by Podochytrium. Mycotaxon 97: 91-109.
  11. ↑ Carlile MJ. 1986. The zoospore and its problems. IN: Ayres, Peter G., and Lynne Boddy, eds. Water, Fungi, and Plants. Vol. 11. Cambridge University Press, 1986.
  12. ↑ Letcher PM et al. 2008. Rhizophlyctidales--a new order in Chytridiomycota. Mycological Research 112: 1031-1048.
  13. ↑ Simmons DR. 2011. Phylogeny of Powellomycetacea fam. Nov. and description of Geranomyces variabilis gen. et comb. nov. Mycologia 103(6): 1411-1420.
  14. ↑ Freeman, K.R. "Evidence that chytrids dominate fungal communities in high-elevation soils". http://www.pnas.org. Retrieved 28 October 2013. External link in |work= (help)
  15. ↑ Lefèvre E, PM Letcher & MJ Powell. 2012. Temporal variation of the small eukaryotic community in two freshwater lakes: emphasis on zoosporic fungi. Aquatic Microbial Ecology 67: 91-105.
  16. ↑ Saving Tiny Toads Without a Home, by Cornelia Dean. The New York Times. February 1, 2010.
  17. ↑ (Voyles, J., L. Berger, S. Young, et al. 2007. Electrolyte depletion and osmotic imbalance in amphibians with chytridiomycosis. Dis Aquat Organ. 77: 113-118.)
  18. ↑ "Chytrid Fungus - causing global amphibian mass extinction". Retrieved 2016-09-06.
  19. ↑ Ibelings BW, de Bruin A, Kagami M, Rijkeboer M, van Donk E. 2004. Host parasite interactions between freshwater phytolankton and chytrid fungi (chytridiomycota). J Phycol 40:457-455.
  20. ↑ Gleason, Frank H., et al. "The ecology of chytrids in aquatic ecosystems: roles in food web dynamics." Fungal Biology Reviews 22.1 (2008): 17-25.
  21. ↑ Hooker WJ. (1981). Compendium of Potato Diseases. International Potato Center. pp. 36–7. ISBN 978-0-89054-027-5.
  22. ↑ "THE CHYTRIDIOMYCOTA". http://website.nbm-mnb.ca. Retrieved 28 October 2013. External link in |work= (help)
  23. ↑ Taylor, T.N.; W. Remy; H. Hass (1994). "Allomyces in the Devonian". Nature. 367 (6464): 601. doi:10.1038/367601a0.
  24. ↑ Krings, Micheal; Nora Dotzler; Thomas Taylor; Jean Galtier (2009). "Microfungi from the upper Visean (Mississippian) of central France: Chytridiomycota and chytrid-like remains of uncertain affinity". Review of Palaeobotany and Palynology. 156 (3-4): 319–328. doi:10.1016/j.revpalbo.2009.03.011.
  25. ↑ Krings, Micheal; Jean Galtier; Thomas N. Taylor; Nora Dotzler (2009). "Chytrid-like microfungi in Biscalitheca cf. musata (Zygopteridales) from the Upper Pennsylvanian Grand-Croix cherts (Saint-Etienne Basin, France).". Review of Palaeobotany and Palynology. 157 (3-4): 309–316. doi:10.1016/j.revpalbo.2009.06.001.
  26. ↑ Michaels, David (2007). Tom Clancy's Splinter Cell: Fallout. Penguin Group. ISBN 978-0-425-21824-2.
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